I love the flavour of this :)

Thank you. I wasn't sure if it should be legendary or not but figured that having more than one on the field would get messy quick with all the tokens hitting the field.

Looks great. Way overcosted though. As a mythic 8/8 I'd expect this to be and probably have trample too. Compare Liege of the Tangle or Godsire for the kind of ridiculousness that 8-mana mythics can do.

I don't know.

I honestly would tweak this in a few ways. I think I would drop "legendary" and "mutant" and maybe even change Hydra to something else since it doesn't use +1/+1 counters. I think I would cost this somewhere between 7-9 and adjust the P/T if necessary, since this is overwhelming in limited and has to have a prohibitive casting cost, but since it's still "just a creature" to removal spells, I don't think it needs to cost 10 or 11.

I'm not worried about it being legendary -- costing 11 already means you can't usually play two of them!

It might need tweaking in a few other ways, currently it's about right on attack -- it discourages blocking without outright preventing it. But it's ridiculously good if it can block anything since it will usually survive and make a lot more blockers. It might be nicer if it focussed on attack more because that's more fun, but it might be fine as is, I'm not sure.

Most of the time creating tokens that make more tokens was too complicated, but with wizards mostly making tokens that have token cards in boosters that's probably ok now, and this is an exciting use of it!

Ok so after looking up hydras in Magic roughly a third of them don't have a starting p/t based on +1/+1 counters, and of those several don't use counters at all. Given the feed back so far I think I can lower the cost to a CMC 8 creature. I'll drop the Mutant and legend types.

Also as a note I took my cue from Mitotic Slime for the tokens that are created.